Pressure sensitive adhesives (PSAs) are used widely as patterns, shapes, and films in a wide variety of industrial and consumer applications. Techniques for creating adhesive patterns are known, including among others flexography, screen printing, spraying, and lamination of die-cut adhesives. The requirements and capabilities of each method are distinct, and have advantages and disadvantages depending on the adhesive material to be coated, the nature of the substrate onto which it is to be coated, the nature of the information to be printed, and the resolution required.
Flexographic printing and screen printing require materials with little or no crosslinking and typical viscosities in the ranges of 100 milliPascal·seconds (mPa·s) to 500 mPa·s and 2000 mPa·s to 5000 mPa·s, respectively. These techniques, which are forms of contact printing, are not well-suited for applications with short print runs due to the investment required to provide a plate or screen as well as the adhesive waste associated with changeover to different print patterns. They also may suffer from excessive adhesive build on the printing element during printing, causing excess adhesive to be printed on the substrate or build on the printing equipment, thereby causing printing defects (e.g., stringing).
Die-cutting of adhesives and subsequent lamination has been used when it is desirable to coat crosslinked, self-supporting, high performance adhesive films onto substrates. An example of this is the manufacture of membrane switches. However, die-cutting of adhesive, followed by lamination is an inherently wasteful and labor intensive step that lends itself to neither high resolution nor variable printing applications due in part to the difficulty and cost of making dies.
Noncontact printing techniques using an ink jet printer, such as ink jet imaging techniques have become very popular in commercial and consumer applications. Ink jet printers operate by ejecting a fluid (e.g., ink) onto a receiving substrate in controlled patterns of closely spaced ink droplets. By selectively regulating the pattern of ink droplets, ink jet printers can produce a wide variety of printed features, including text, graphics, images, holograms, and the like. Moreover, ink jet printers are capable of forming printed features on a wide variety of substrates, including not just flat films or sheets, but also three-dimensional objects as well.
Thermal ink jet printers and piezo ink jet printers are the two main types of ink jet systems in widespread use today. For both approaches, the jetted fluid should meet stringent performance requirements in order for the fluid to be appropriately jettable and for the resultant printed features to have the desired mechanical, chemical, visual, and durability characteristics. In particular, fluids should have relatively low viscosity when passing through the printhead, yet should be able to form accurate, durable images on the desired receiving substrate. For example, a typical fluid for thermal ink jetting typically has a viscosity in the range of 3 to 5 mPa·s measured at a shear rate of 1000 s−1 at 25° C., while piezo ink jet fluids typically have a viscosity in the range of 1 to 30 mPa·s measured at a shear rate of 1000 s−1 at the printhead temperature. The need to use low viscosity fluids (e.g., inks) may make it challenging to obtain printed features with sufficient thickness and resolution to achieve good mechanical, chemical, visual, adhesion, and durability characteristics.
Organic solvent-based and water-based jettable fluids are well known. A typical water-based fluid generally comprises water, optionally one or more organic co-solvents, and optionally one or more additives that are included to enhance the desired performance of the fluid. Representative examples of such additives include one or more colorants, slip modifiers, thixotropic agents, tack promoting agents, tack reducing agents, foaming agents, antifoaming agents, flow or other rheology control agents, waxes, oils, plasticizers, binders, antioxidants, fungicides, bactericides, organic and/or inorganic filler particles, leveling agents, opacifiers, antistatic agents, dispersants, and the like.
Water-based fluids have drawbacks. For industrial applications, drying is energy and equipment intensive. Drying water also takes time, and the printed material needs to be handled carefully during the relatively lengthy drying period. Water-based fluids also are compatible only with a limited range of substrates, typically those on which the water is absorbed to some degree. Images formed using water-based fluids typically require a protective overlaminate for outdoor applications.
Instead of water, other solvent-based fluids include relatively volatile, organic solvents. Such fluids dry more rapidly and easily than aqueous fluids. However, such solvents may be toxic, flammable, or the like, requiring careful handling. These fluids also tend to be compatible with only a limited range of substrates.
In order to avoid using a conventional solvent, fluid compositions incorporating a free radically polymerizable, fluid (e.g., monomers) have been developed. The fluid not only functions as a solvent, but also functions as a viscosity reducer, as a binder when cured, and optionally as a crosslinking agent. In the uncured state, these compositions have a low viscosity and are readily jettable. However, the monomers readily crosslink upon exposure to a suitable source of curing energy, e.g., ultraviolet light, electron beam energy, heat, and/or the like, to form a crosslinked polymer network.
Printed, especially ink jet printed compositions, also require good dot gain characteristics. Dot gain refers to the degree to which a printed dot spreads upon application to a substrate. If a printed feature (e.g., a dot or line) spreads too much on the substrate, the resultant image may tend to have poor resolution. On the other hand, if a printed feature spreads insufficiently upon application to the substrate, poor image density may result.
Ink jetting theoretically would allow PSAs to be printed onto a wide range of substrates to form high resolution adhesive patterns. However, inherently sticky materials are difficult to jet with any success. Firstly, to jet properly, the material should have low elasticity and low viscosity when jetted and should not have a tendency to plug up the orifices of the printhead. Inherently sticky materials fail to satisfy these criteria.
Additionally, the material should hold itself together without too much dot gain. If the material flows away from the targeted area too readily, forming accurate adhesive patterns is not possible. Further, the printed material should be thick enough to build sufficient z-axis height to function as a PSA. Unfortunately, materials that jet readily through ink jet printheads have low viscosities, flow readily, and generally do not satisfy these capabilities.
In particular, it is generally important to build print thickness of PSAs in order to realize the desired adhesive properties. Using inkjet technology, printed dot sizes of 30 to 50 micrometers diameter may be routinely achieved. However, it is very difficult to achieve the desired thickness (i.e., greater than about 10 micrometers) without a simultaneous loss of resolution due to dot spreading. That is, repeated overprinting of adhesive dots causes the material in the drops to spread sideways on the substrate, enlarging the dot size.
PCT Publication No. WO 95/15266 A describes ink jet printing of radiation curable composition having adhesive properties. However, such composition does not form a PSA when cured, but rather a non-tacky film or other cured body. The document also does not describe incorporating any kind of rheology modifying agent into the composition in order to control characteristics such as viscosity, dot gain, thixotropy, and/or the like.
Patterned application of high performance PSAs (i.e., those PSAs having high tack, high bond strength, and low creep) is important to the manufacture of laminated devices such as, for example, membrane switches and graphic articles such as substrateless labels and graphics. It would be desirable to have a method such as ink jetting that can variably print PSAs by a non-contact method onto a wide range of substrates in a wide range of patterns.